I put my flaps on after installing them. Doesn't look bad. And I hope it helps with reversion which is a problem with some of the pipes out there. Easy to put on so I'd do it. By the way, mine brought my decibel level from 96 to 85 . . . quite a feat!
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http://www.hotboatpics.com/pics/data...G_0025-med.JPG